OKIGIN OF THE PACIFIC MOUNTAINS 131 



Wrangell and Nutzotin mountains. An inspection of the map will make 

 these relations clear to the reader, and open the way to the suggestion 

 that the overlapping of mountain axis, which is general throughout 

 Alaska, may be the direct result of varying degrees of uplift in adjacent 

 portions of parallel belts of the earth's crust, separated one from another 

 by zones of sharp flexure or faulting. Along the axis of any one of these 

 belts the deformation is ordinarily gentle but extensive, while across the 

 belts the breaks are sharply localized. 



It is anticipated that the work of the United States Geological Survey 

 now in progress in Alaska will add greatly to existing knowledge of the 

 orography of the northern portions of the Pacific mountains. 



Geologic Dates 



It is to Dr G. M. Dawson that we are indebted for the earliest recogni- 

 tion of the baselevel character of the Interior plateau of British Columbia, 

 Yukon, and Alaska, and the age relations of the peneplain have been 

 fully discussed by this geologist, who assigns it to erosion continued 

 through Eocene time. In British Columbia the Interior plateau became 

 the seat of a series of fresh water lakes through warping of the surface, 

 which was initiated about the close of the Eocene. In the basins thus 

 formed Miocene sediments were deposited in alternation with beds of 

 fragmental volcanic material and sheets of lava. The descriptions of 

 Dawson refer particularly to the portion of the plateau which extends 

 northward from the United States boundary to where the British pos- 

 sessions join Alaska. Of those by whom it has been noted and described 

 within the territory of Alaska, Spurr* alone has found reason to differ 

 with the date which Dawson has assigned to the feature. This geologist 

 shows that the erosion of the Yukon plateau was contemporaneous with 

 the deposition of the Miocene strata in the lower valley of the Yukon 

 river, and from this argues that the Yukon plateau was eroded in Mio- 

 cene time. It seems to the writer that the discrepancy between the con- 

 clusions of the two authors cited is more apparent than real, for, in the 

 absence of evidence to the contrary, it may be true that the reduction 

 of the peneplain was largely accomplished during the Eocene, even in 

 the region where it has been described as extending into the Miocene. 

 Indeed, a recurrence of planation after the deposition and folding of 

 Miocene strata is recognized by Dawson in British Columbia. 



The development of the antecedent river systems and the production 

 of the peneplain were contemporaneous, and the latter therefore date 



*Josiah Edward Spurr : Geology of the Yukon Gold district, Alaska. Eighteenth Ann. Rept. 

 U. S. Geol. Survey, pt. iii, 1898, p. 260. 



